In recent years, applicable fields of liquid crystals such as various kinds of display devices, electronic optical devices, liquid crystal sensors, etc. have markedly been enlarged, and accompanying these situation, liquid crystal compounds having various structures have been proposed. In liquid crystal materials particularly used for display devices, nematic liquid crystals are at present in main stream, and a TN type or a STN type simple matrix system using the same and a TFT type active matrix system in which a thin film transistor is provided to respective picture elements have been used. However, a driving force of the nematic liquid crystal is based on weak interaction between anisotropy of dielectric constant of a liquid crystal material and an electric field so that it has a drawback that a response speed is essentially late (msec order). Thus, it is disadvantageous as a material for a display device with a large sized screen in which high speed response is required.
To the contrary, a ferroelectric liquid crystal which has firstly been synthesized by R. B. Meyer et al. in 1975 has a spontaneous polarization and this acts directly on an electric field so that the liquid crystal has a large driving force. Since N. A. Clark et al. has reported in 1980 about a high speed response with micro-second order and memory effect of a surface stabilized ferroelectric liquid crystal device (SSFLCD), it has been attracted to attention and many ferroelectric liquid crystal compounds have been synthesized.
When the ferroelectric liquid crystal compound is used as a material for a display device, the following conditions are generally required. (1) It has a chiral smectic C phase (SmC* phase) in a wide temperature range including room temperature. (2) Electro-optic response speed is high. (3) Alignment quality is good. Until now, it is difficult to satisfy all the conditions with a single compound.
Accordingly, there has been employed a method in which several kinds of compounds having SmC* phases are mixed or an optically active compound is added to an achiral host mixture having a smectic C phase (SmC phase) which has a low viscosity whereby a ferroelectric liquid crystal composition having desired characteristics and exhibiting SmC* phase is obtained.
In the latter case, a chiral dopant to be added may, itself, have BmC* phase or may not have the phase, and it is required to have good compatibility with the achiral host mixture, induce a high magnitude of spontaneous polarization and not increase the viscosity.
It is well known that a response time of a ferroelectric liquid crystal is expressed by the following equation; .tau.=.eta./(Ps.E). Here, .eta. represents a rotational viscosity, Ps represents a spontaneous polarization, and E represents an electric field intensity. From this equation, in order to obtain a high speed response, a liquid crystal material having a low viscosity and a large spontaneous polarization has been aimed to be developed.
The spontaneous polarization has been considered to be caused by the result that free rotation of a dipole moment perpendicular to a molecular long axis is restricted around it by the effect of an asymmetric carbon. Accordingly, in order to increase spontaneous polarization, many attempts have been made by the methods that (1) a dipole portion is allowed to come near a skeleton portion which is a so-called core, (2) a dipole portion and an asymmetric carbon are allowed to place near position, and (3) a sterically large substituent is attached to an asymmetric carbon whereby free rotation around a molecular long axis is restricted, etc.
Further, it has recently been reported that a compound having a structure that a dipole portion and an asymmetric carbon are directly bonded to a 5-membered lactone effectively inhibits free rotation whereby having a large spontaneous polarization (Japanese Journal of Applied Physics, vol. 29, No. 6, ppL 981 to L 983).
When a ferroelectric liquid crystal mixture is aligned by a rubbing method which has conventionally been carried out in a nematic liquid crystal, its alignment quality is different depending on a phase sequences of a liquid crystal material, and it is preferred to have the phase sequences of an isotropic phase (Iso phase).fwdarw. a cholesteric phase (N* phase).fwdarw. a smectic A phase (SmA phase).fwdarw. a chiral smectic C phase (SmC* phase).
Here, in the N* phase, it is necessary to have a sufficiently long helical pitch.
Thus, in order to unwind the helix in the N* phase or the SmC* phase, there has been employed a method of elongating the helical pitch by mixing a ferroelectric liquid crystal having a reverse helical sense (Japanese Patent Applications Laid-Open No. 117488/1992 and No. 220289/1991).